Push-pull fpga cell

ABSTRACT

A flash memory cell includes a p-channel flash transistor having a source, a drain, a floating gate, and a control gate, an n-channel flash transistor having a source, a drain coupled to the drain of the p-channel flash transistor, a floating gate, and a control gate, a switch transistor having a gate coupled to the drains of the p-channel flash transistor and the n-channel flash transistor, a source, and a drain, and an n-channel assist transistor having a drain coupled to the drains of the p-channel flash transistor and the n-channel flash transistor, a source coupled to a fixed potential, and a gate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/334,059, filed Dec. 12, 2008, the entirety of which isincorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to non-volatile memory cells. Moreparticularly, the present invention relates to push-pull non-volatilememory cells and to arrays of such memory cells.

2. The Prior Art

Push-pull non-volatile memory cells are known in the art. FIG. 1 is aschematic diagram shows an illustrative prior-art push-pull non-volatilememory cell. The memory cell includes a p-channel non-volatile memorytransistor connected in series with an n-channel non-volatiletransistor. The p-channel non-volatile memory transistor and then-channel non-volatile transistor may be fabricated as floating-gateflash transistors or may be fabricated using other known non-volatiletransistor technologies.

The memory cell shown in FIG. 1 may be used to drive a switch transistorsuch as the n-channel transistor shown with its gate coupled to thecommon drain connections of the p-channel non-volatile memory transistorand the n-channel non-volatile transistor. Such a memory cellarrangement may be used to form programmable circuit connections in aprogrammable integrated circuit such as a field programmable gate array(FPGA) integrated circuit.

The memory cell shown in FIG. 1 may be programmed into one of twostates. In the first state, the n-channel non-volatile transistor isturned on and the p-channel non-volatile transistor is turned off. Inthis state, the gate of the n-channel switch transistor is grounded,turning it off. In the second state, the p-channel non-volatiletransistor is turned on and the re-channel non-volatile transistor isturned off. In this state, the gate of the n-channel switch transistoris at approximately V_(DD), turning it on.

In the case of flash non-volatile memory transistors, the p-channelnon-volatile transistor is programmed by placing a negative voltage,such as −4v, on the source of the p-channel non-volatile transistor,placing a positive voltage such as 8.5v on its gate, while its bulk isbiased at a voltage such as 1.2v. The gate and source of the p-channelnon-volatile transistor are biased at 0v during this procedure.

The n-channel flash non-volatile transistor is programmed by placing avoltage such as 8.5v, on its gate, and placing a voltage such as 4.5v onits source. The gate of the p-channel non-volatile transistor is biasedat a voltage such as −3.3v and its source is biased at a voltage such as0v during this procedure.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a schematic diagram of an illustrative prior-art push-pullnon-volatile memory cell.

FIG. 2 is a schematic diagram of an illustrative push-pull non-volatilememory cell according to one aspect of the present invention.

FIG. 3 is a schematic diagram of a portion of an illustrative array ofpush-pull non-volatile memory cells according to one aspect of thepresent invention.

FIG. 4 is a schematic diagram of an illustrative push-pull non-volatilememory cell according to one aspect of the present invention.

FIG. 5 is a schematic diagram of a portion of an illustrative array ofpush-pull non-volatile memory cells according to one aspect of thepresent invention.

DETAILED DESCRIPTION

Persons of ordinary skill in the art will realize that the followingdescription of the present invention is illustrative only and not in anyway limiting. Other embodiments of the invention will readily suggestthemselves to such skilled persons.

Referring now to FIG. 2, a schematic diagram shows an illustrativepush-pull non-volatile flash memory cell 10 according to one aspect ofthe present invention. Memory cell 10 includes a p-channel flashtransistor 12 connected in series with an n-channel flash transistor 14.An n-channel switch transistor 16 has its gate connected to the commondrain connections of the p-channel flash transistor 12 and the n-channelflash transistor 14. The n-channel switch transistor may be used to makeprogrammable connections between circuit nodes coupled to its source anddrain to form programmable circuits as is known in the art. Anadditional n-channel assist transistor 18 has its drain connected to thecommon drain connections of the p-channel flash transistor 12 and then-channel flash transistor 14 and its source connected to ground. Thep-channel non-volatile memory transistor and the n-channel non-volatiletransistor are disclosed in the specific example herein as employingflash technology, but persons of ordinary skill in the art willappreciate that they may be fabricated as floating-gate flashtransistors or may be fabricated using other known non-volatiletransistor technologies.

To program p-channel flash transistor 12, a voltage such as 8.5v isapplied to its control gate, a negative voltage such as −4v is appliedto its source, and a voltage such as 1.2 v is applied to the n-well inwhich p-channel flash transistor 12 is formed. Ground potential (0v) isapplied to the control gate of n-channel flash transistor 14 and to thegate of n-channel assist transistor 18. The drain of n-channel flashtransistor 14 may be left floating. The p-channel flash transistor isprogrammed using band-to-band (BTB) programming.

To program n-channel flash transistor 14, a voltage such as 8.5v isapplied to its control gate, and a voltage such as 4.5v is applied toits source. Ground potential (0v) is applied to the source of thep-channel flash transistor 12 and a negative voltage such as −3.3v isapplied to its control gate. A voltage such as 3.3v is applied to thegate of n-channel assist transistor 18. The n-channel flash transistor14 is programmed using hot carrier injection (HCl) programming. Then-channel assist transistor 18 in the memory cell 10 is employed to passHCl programming current for programming the n-channel flash transistor14 in the cell. The programming voltages suggested for programmingtransistors 12 and 14 may scale depending on feature size and/or thememory technology employed. Persons of ordinary skill in the art willreadily appreciate what voltages to employ to program memory transistors12 and 14 in any given non-volatile memory technology.

Referring now to FIG. 3, a schematic diagram shows a portion 20 of anillustrative array of push-pull non-volatile memory cells according toone aspect of the present invention. The portion 20 of the array shownin FIG. 3 includes memory cells 10-1 and 10-2 in a first row of thearray, and memory cells 10-3 and 10-4 in a second row of the array.Memory cells 10-1 and 10-3 are in a first column of the array and memorycells 10-2 and 10-4 are in a second column of the array.

The control gates of p-channel flash transistors 12-1 and 12-2 in memorycells 10-1 and 10-2 in the first row of the array are connected togetherto a p-channel row line 22. The control gates of n-channel flashtransistors 14-1 and 14-2 in memory cells 10-1 and 10-2 are connectedtogether to an n-channel row line 24. The gates of n-channel assisttransistors 18-1 and 18-2 in memory cells 10-1 and 10-2 are connected toa row line 26. The control gates of p-channel flash transistors 12-3 and12-4 in memory cells 10-3 and 10-4 in the second row of the array areconnected together to a p-channel row line 28. The control gates ofn-channel flash transistors 14-3 and 14-4 in memory cells 10-3 and 10-4are connected together to an n-channel row line 30. The gates ofn-channel assist transistors 18-3 and 18-4 in memory cells 10-3 and 10-4are connected to a row line 32.

The sources of p-channel flash transistors 12-1 and 12-3 in memory cells10-1 and 10-3 in the first column of the array are connected together toa p-channel column line 34. The sources of n-channel flash transistors14-1 and 14-3 in memory cells 10-1 and 10-3 are connected together to ann-channel column line 36. The sources of p-channel flash transistors12-2 and 12-4 in memory cells 10-2 and 10-4 in the second column of thearray are connected together to a p-channel column line 38. The sourcesof n-channel flash transistors 14-2 and 14-3 in memory cells 10-2 and10-4 are connected together to an n-channel column line 40.

When programming the p-channel flash transistors in the array, thep-channel row lines containing cells that are unselected are driven to0v. The p-channel and n-channel column lines containing cells that areunselected are driven to a voltage that will minimize gate disturb ofthe memory cells on the unselected column lines, such as about 2-5volts. When programming the n-channel flash transistors in the array,the n-channel row lines containing cells that are unselected are drivento 0v. The p-channel and n-channel column lines containing cells thatare unselected are driven to a voltage that will minimize gate disturbof the memory cells on the unselected column lines, such as about 2-5volts.

To simultaneously erase both the p-channel and n-channel flashtransistors in all of memory cells 10-1 through 10-4, a negative voltagesuch as −8.5 volts is applied to all of row lines 22, 24, 26, 28, 30,and 32, p-channel and n-channel column lines 34, 36, 38, and 40 areallowed to float, and a voltage such as 1.2v is applied to the sourcesand drains of switch transistors 16-1 through 16-4.

The erase mechanism of the memory cells of the present invention isFowler-Nordheim tunneling (FN). Because of this, the p-channel andn-channel flash transistors 12 and 14 need coupling ratios that areabout 10% better than normal flash cells because the p-well/n-well biascan not be more than the switch transistor oxide tolerance level. Thep-channel and re-channel flash transistor size is independent whenprogramming using HCl because the majority program current is passedfrom the n-channel assist transistors 18.

The memory cell of FIG. 2 has the advantage that an independent read ofthe status of the n-channel and p-channel transistors 12 and 14 isperformed without disturbing the other one of the transistors 12 and 14.Reading one of the n-channel and p-channel transistors 12 and 14 usingtransistor 18 does not require driving the gate of the other one ofn-channel and p-channel transistors 12 and 14.

Referring now to FIG. 4, a schematic diagram shows an illustrativepush-pull non-volatile memory cell 50 according to another aspect of thepresent invention. Memory cell 50 is almost identical to memory cell 10of FIG. 2 and includes a p-channel flash transistor 12 connected inseries with an n-channel flash transistor 14. An n-channel switchtransistor 16 has its gate connected to the common drain connections ofthe p-channel flash transistor 12 and the n-channel flash transistor 14.An additional n-channel assist transistor 18 has its drain connected tothe common drain connections of the p-channel flash transistor 12 andthe n-channel flash transistor 14 and its source connected to ground.The difference between memory cell 10 of FIG. 2 and memory cell 50 ofFIG. 4 is that p-channel flash transistor 12 and the n-channel flashtransistor 14 in memory cell 50 of FIG. 4 share a common floating gate.

One advantage of the memory cell 50 of FIG. 4 is that only the n-channelflash transistor 14 needs to be erased and programmed since it sharesits floating gate with the p-channel flash transistor 12. Thus theprogramming circuitry that needs to be provided is less complicatedsince it only has to be used to provide the voltages necessary toprogram the re-channel flash transistor 14.

Referring now to FIG. 5, a schematic diagram shows a portion 60 of anillustrative array of push-pull non-volatile memory cells according toanother aspect of the present invention. The portion 60 of the arrayshown in FIG. 5 uses the memory cell depicted in FIG. 4 in which thep-channel and n-channel flash transistors share a common floating gate.

The portion 60 of the memory cell array shown in FIG. 5 includes memorycells 50-1 and 50-2 in a first row of the array, and memory cells 50-3and 50-4 in a second row of the array. Memory cells 50-1 and 50-3 are ina first column of the array and memory cells 50-2 and 50-4 are in asecond column of the array.

The control gates of p-channel flash transistors 12-1 and 12-2 in memorycells 50-1 and 50-2 in the first row of the array are connected togetherto a row line 22. The control gates of n-channel flash transistors 14-1and 14-2 in memory cells 10-1 and 10-2 are also connected together top-channel row line 22. Separate row lines for the control gates of eachof the p-channel and n-channel flash transistors seen in the embodimentof FIG. 3 are not used. The gates of n-channel assist transistors 18-1and 18-2 in memory cells 10-1 and 10-2 are connected to a row line 26.The control gates of p-channel flash transistors 12-3 and 12-4 andn-channel flash transistors 14-3 and 14-4 in memory cells 10-3 and 10-4in the second row of the array are connected together to a row line 28.The gates of n-channel assist transistors 18-3 and 18-4 in memory cells10-3 and 10-4 are connected to a row line 32. The row lines areconnected to row-access circuitry as is known in the art.

The sources of p-channel flash transistors 12-1 and 12-3 in memory cells10-1 and 10-3 in the first column of the array are connected together toa p-channel column line 34. The sources of n-channel flash transistors14-1 and 14-3 in memory cells 10-1 and 10-3 are connected together to ann-channel column line 36. The sources of p-channel flash transistors12-2 and 12-4 in memory cells 10-2 and 10-4 in the second column of thearray are connected together to a p-channel column line 38. The sourcesof n-channel flash transistors 14-2 and 14-3 in memory cells 10-2 and10-4 are connected together to an n-channel column line 40.

To program memory cells in the array of FIG. 5, the p-channel columnlines 34, and 38 are allowed to float. The ones of the n-channel columnlines 36 and 40 for columns not containing the memory cell to beprogrammed are also allowed to float. Only the n-channel column line forthe column containing the memory cell to be programmed is driven to avoltage such as 4.5v. The one of lines 22 and 26 for the row containingthe memory cell to be programmed is driven to a voltage such as 8.5v.The other row lines in the array are driven to 0v. The one of row lines30 and 32 containing the memory cell to be programmed is driven to avoltage such as 3.3v. The other ones of row lines 30 and 32 notcontaining the memory cell to be programmed may driven to a voltage suchas either 0v or 3.3v. The mechanism used to program the selected memorycell is HCl programming.

In order to simultaneously erase all of the memory cells in array 50,all of the column lines 34, 36, 38, and 40 are allowed to float. Avoltage such as 7v is applied to the wells containing the n-channel andp-channel flash transistors. A negative voltage such as −8.5v is appliedto the control gates of the n-channel and p-channel flash transistors onrow lines 22 and 26. A voltage such as 1.2v is applied to the sourcesand drains of the switch transistors, and a voltage such as 3.3v isapplied to the gates of the n-channel assist transistors on row lines 30and 32.

During normal operation, 0v is applied to the gates of the n-channelassist transistors 18-1 through 18-4 to turn them off. A voltage such as3.3v is applied to p-channel column lines 34 and 38, and 0v is appliedto n-channel column lines 36 and 40. A voltage such as 3.3v is appliedto the control gates of the n-channel and p-channel flash transistors onrow lines 22 and 26.

While embodiments and applications of this invention have been shown anddescribed, it would be apparent to those skilled in the art that manymore modifications than mentioned above are possible without departingfrom the inventive concepts herein. The invention, therefore, is not tobe restricted except in the spirit of the appended claims.

1. A flash memory cell including: a p-channel flash transistor having asource, a drain, a floating gate, and a control gate; an n-channel flashtransistor having a source, a drain coupled to the drain of thep-channel flash transistor, a floating gate, and a control gate; aswitch transistor having a gate coupled to the drains of the p-channelflash transistor and the n-channel flash transistor, a source, and adrain; and an n-channel assist transistor having a drain coupled to thedrains of the p-channel flash transistor and the n-channel flashtransistor, a source coupled to a fixed potential, and a gate.